I am merely stating that this is an aspect which I feel was left hanging in the movie and might have been answered in the comics. But I can't know for sure if thats the case since I have no background in the comics.
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KeyBrd BasherApr 26 '13 at 10:21

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I still feels its better suited on scifi and even flagged the mod too. Because their is no explanation of it in movie.
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Ankit Sharma♦Apr 26 '13 at 10:44

2 Answers
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This answer is from the comics, as they don't show it in the movie. In the comics, classic Green Lantern had to recharge the ring every 24 hours. This (and a weakness to yellow objects) was his kryptonite; that crucial weakness that prevents him from being an invincible god. So, it would be common for fights to go longer than Hal expected, resulting in him running out of charge mid-fight. Then Hal is just a regular guy (but the uniform would stay, even though it's generated by the ring...), and had to use his wits and environment to defeat his foe.

Modern Green Lantern (since roughly the year 2000), instead has the ring be more like an electronic gadget. He charges the ring, and the more he uses it, the faster it drains, until it runs out of power. When he charges it off the lantern, it returns it to maximum charge.

Note that the rings are capable of providing their charge level upon request. So during long battles, you'll see Green Lanterns checking their rings' charge. The modern rings also result in situations where a stranded Lantern will set things up so they're in a low-power-usage state. For example, if they're in space and unable to reach their lantern in time to recharge, they can have the ring put themselves into hibernation, provide a life support bubble, and just drift toward their destination while sending an occasional distress signal. This then helps the Lantern survive until they can be rescued and get back to a lantern to charge their ring.

Just to add a couple of comments - the 'newer' version of the ring came about some time before Kyle Rayner became Green Lantern. One of the plot points of Emerald Twilight, the story of Hal going insane and attacking all the Green Lanterns and The Guardians, is that Hal seems to become more powerful with each new ring he adds (though it's also suggested this is purely psychological). Also, there was a story where Kyle tried 'recruiting' his own heroes, and one of them used the last reserve to help his allies - at the cost of his own life (I don't remember the name of that story)
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Barry HammerMay 21 '13 at 3:26

Back when the rings needed to be charged every 24 hours, the rings had an emergency power reserve specifically to protect the bearer from mortal harm when their ring's regular charge ran out. (See Green Lantern v2 #46 (1966).)